Ultrashort filaments of light in weakly ionized, optically transparent media

Abstract

Modern laser sources nowadays deliver ultrashort light pulses reaching few cycles in duration and peak powers exceeding several terawatt (TW). When such pulses propagate through optically transparent media, they first self-focus in space and grow in intensity, until they generate a tenuous plasma by photo-ionization. For free electron densities and beam intensities below their breakdown limits, these pulses evolve as self-guided objects, resulting from successive equilibria between the Kerr focusing process, the chromatic dispersion of the medium and the defocusing action of the electron plasma. Discovered one decade ago, this self-channeling mechanism reveals a new physics, widely extending the frontiers of nonlinear optics. Implications include long-distance propagation of TW beams in the atmosphere, supercontinuum emission, pulse shortening as well as high-order harmonic generation. This review presents the landmarks of the 10-odd-year progress in this field. Particular emphasis is laid on the theoretical modeling of the propagation equations, whose physical ingredients are discussed from numerical simulations. The dynamics of single filaments created over laboratory scales in various materials such as noble gases, liquids and dielectrics reveal new perspectives in pulse shortening techniques. Far-field spectra provide promising diagnostics. Attention is also paid to the multifilamentation instability of broad beams, breaking up the energy distribution into small-scale cells along the optical path. The robustness of the resulting filaments in adverse weathers, their large conical emission exploited for multipollutant remote sensing, nonlinear spectroscopy and the possibility of guiding electric discharges in air are finally addressed on the basis of experimental results.

Optimal laser-pulse energy partitioning for air ionization

“…As our interest is atmospheric applications, we consider the case of a collimated or loosely focused beam. In this case, the individual filaments of the multiple filamentation pattern have similar properties, including ionization level, than a single filament [15][16][17]. This assumption would not hold in the case of tightly focused beams, where the ionization and the energy deposition increases [43,44].…”

Regularizing Aperiodic Cycles of Resonant Radiation in Filament Light Bullets

“…This regime is known to exhibit much more complicated dynamics but also to include the advantage of solitary solutions with full spatiotemporal localization and stationarity, known as light bullets [18][19][20][21][22][23][24][25][26]. In a filament, these conditions cannot be exactly fulfilled [1,2] due to a high number of inherent perturbations [27], but the observations in Ref. [17] demonstrate unexpected long light-bullet propagation and reveal also another interesting point.…”

“…The filamentation of intense femtosecond laser pulses in transparent media represents, due to the interplay of basic linear and nonlinear effects resulting from classical and quantum mechanical approaches, a physical system of interest for fundamental science as well as for a variety of applications [1][2][3][4]. It is used for the investigation of, e.g., supercontinuum [5], high harmonic [6], or terahertz generation [7], few-cycle pulse generation [8], or complex nonlinear dynamics as self-organization [9].…”

Interpretation of negative birefringence observed in strong-field optical pump-probe experiments: High-order Kerr and plasma grating effects

“…In the HOK model, where the nonlinear refractive index is defined by n Kerr = n 2 I + n 4 I 2 + n 6 I 3 + n 8 I 4 , the saturation and inversion of the electronic Kerr index are due to negative n 4 and n 8 terms. It is well known that the optical Kerr effect plays a central role in the nonlinear propagation of short and intense laser pulses such as selfphase modulation, self-focusing, pulse compression, and laser filamentation [9][10][11][12][13]. The potential impact of the HOK model on the last process was presented in Refs.…”